A number of biological solid wastes or biological solid materials such as wastewater sludge, pharmacy fungi residues, algae residues, food industry residues, fermentation residues, coals with high moisture content, and agricultural residues are currently produced each year in China, with large productions. The production of only sludge from municipal sewage plants each year is 30 million tons, the annual emission of fungi residues of antibiotic byproducts reaches above 1.8 million tons, and the amount of algae residues is above 1 million tons. These biological solid wastes have high moisture content and large volume, and require dewatering treatment to reduce the volume.
Depending on different moisture removal manners, the dewatering method of biological solid materials may be divided into two main types: evaporative drying and non-evaporative dewatering.
Evaporative drying is to achieve drying of sludge by moisture evaporation, for example, anhydration drying. Anhydration is a treatment process for rapidly evaporating moisture in sludge with heat energy, and depending on the source of heat energy and the heating method, may be divided into fluidzing drying, superheated steam drying, dividing wall drying, infrared radiation drying, impinging stream drying, and so on. The advantages of anhydration drying technique are that during the drying, sludge is stable in properties, not easy to cohere and not easy to generate marsh gas, gases generated are nonflammable and less likely to explode, and the sludge moisture content after drying is lower than 10%. The disadvantages include large investment, high operation cost, and high technical demands in management and operation.
Non-evaporative dewatering mainly includes mechanical dewatering methods such as centrifugation and press filtration and non-conventional dewatering methods representative of solvent replacement. Mechanical dewatering uses a pressure difference between both sides of a filtering medium as a driving force such that moisture in a biological solid material is forced through the filtering medium to be removed in a liquid form. If the moisture content is decreased to 80% within a short time after sludge is subjected to mechanical dewatering, but is still high, subsequent treatment and disposal may be difficult. Thus, other deep dewatering or dewatering techniques are required for further dewatering.
Solvent extraction dewatering: the purpose is to not consume the latent heat of evaporation of water and not evaporate moisture in a biological solid material, moisture is transferred from sludge to a solvent by means of dissolution, absorption or exchange by mixing and interaction of the solvent and the sludge, and then the biological solid material is separated from the aqueous solvent and water is directly removed in a liquid state.
Microwave in treatment of a biological solid material can improve dewatering property and degradability of the biological solid material, and microwave in a liquid phase generates thermal effect and sonochemical effect that break floc, zooglea and cell bodies of the biological solid material such that bound water in the biological solid material is converted to free water. Microwave assisted solvent extraction dewatering method is efficient and stable, and has enormous application potential.